The present invention relates to a method of multi-stage combustion of sulfur- and nitrogen-bearing coal which provides high combustion efficiency and at the same time prevents formation of the air pollutants SO.sub.x and NO.sub.x (oxides of sulfur and nitrogen.)
It is known that during conventional combustion of fossil fuels, the nitrogen and sulfur chemically bound in those fuels can be oxidized to NO.sub.x and SO.sub.x, respectively. In addition, NO.sub.x can be formed by high temperature oxidation of nitrogen in the combustion air. If these acid gases, NO.sub.x and SO.sub.x, are released to the atmosphere, they can be absorbed in atmospheric moisture and thereafter precipitate to earth as acid rain.
The prior art discloses various processes under which nitrogen-bearing fuels and fuels bearing both nitrogen and sulfur can be burned, generally under fuel-rich conditions, to limit emissions of NO.sub.x and SO.sub.x. Examples of prior art patents are:
______________________________________ Blair et al 4,343,606 Carver et al 4,381,718 Ashworth et al 4,395,975 Ashworth et al 4,423,702 Dykema 4,427,362 Adrain et al 4,475,472 Moriarty 4,517,165 Moriarty et al 4,523,532 Brown et al 4,542,704 Brown 4,582.005 Breen et al 4,779,545 Dykema 4,807,542 Bell 4,951,579 Dykema 5,085,156 ______________________________________
None of the prior art teaches the need for an initial very high temperature combustion regime to free sulfur from its bound form in the coal. Neither does the prior art teach the need for both very high and low temperature regimes, together, to first free the sulfur from the coal and then to capture it with alkali materials, to optimize sulfur release and capture. In addition, none of the prior art teaches separation of all or part of the oxygen and nitrogen in the combustion air to provide both oxygen-rich and oxygen-lean combustion air, to sequentially raise and lower combustion temperatures, respectively, as required for optimum control of both SO.sub.x and NO.sub.x.
It is an object of the present invention to efficiently and effectively control emissions of the oxides of both nitrogen (NO.sub.x) and sulfur (SO.sub.x) to atmosphere from processes involving combustion of coal with air.
Another object is the provision of a coal combustion process of the above type which not only controls the above pollutants but results in highly efficient fuel combustion and environmentally benign solid wastes as well.
A still further object is to provide a process as noted above which achieves clean coal combustion, wherein coal is burned to release heat for useful work and particularly to generate steam and electricity.